ASTORIA, Ore.  To protect the marbled murrelet, logging stopped across wide swaths of coastal old-growth forests in Oregon. But a new study indicates the small seabird may have been forced into decline not just by logging, but also by overfishing.

Fleets that hauled sardines from the West Coast may have forced the murrelet to rely on poorer quality food, undermining their breeding, scientists from the University of California at Berkeley say.

If correct, the theory may explain the dwindling of other populations of coastal birds, a point the researchers are now beginning to examine.

"It looks like a lot of the more energetically valuable prey are less available to those seabirds now," said Ben Decker, lead author of the murrelet study which is to be published this year in the scientific periodical, Conservation Biology.

The study suggests that fishery management decisions need to consider the effects on other species, the scientists said.

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Since 1992, the murrelet has been designated a threatened species in Washington, California and Oregon, mainly because of declines blamed on logging. That protection, along with those for the northern spotted owl, brought logging to a halt on many federal lands in the Northwest.

The study does not contradict the earlier assumption that logging has hurt the murrelet by eroding its habitat. Rather, the research shows that the seabird probably endured a "double whammy" from the slashing of its habitat and erosion of its prey, Steve Beissinger, a professor of conservation biology at University of California at Berkeley, told The Oregonian.

The scientists reached their conclusion after examining the chemical makeup of feathers from 136 murrelets in museum collections dating from 1895 to 1911, before the arrival of large West Coast fisheries. They compared those feathers with the plumage from 201 modern birds.